The structure and mode of division of the nucleus of budding yeast cells have been studied by phase-contrast microscopy during life and by ordinary microscopy after Helly fixation. The components of the nucleus were differentially stained by the Feulgen procedure, with Giemsa solution after hydrolysis, and with iron alum haematoxylin. New information was obtained in ceils fixed in Helly's by directly staining them with 0.005% acid fuchsin in 1% acetic acid in water. Electron micrographs have been made of sections of cells that were first fixed with 3% glutaraldehyde, then divested of their walls with snail juice, and postfixed with osmium tetroxide. Light and electron microscopy have given concordant information about the organization of the yeast nucleus. A peripheral segment of the nucleus is occupied by relatively dense matter (the ':peripheral duster" of Mundkur) which is Feulgen negative. The greater part of the nucleus is filled with fine-grained Feulgen-positive matter of low density in which chromosomes could not be identified. Chromosomes become visible in this region under the light microscope at meiosis. In the chromatin lies a short fiber with strong affinity for acid fuchsin. The nucleus divides by elongation and constriction, and during this process the fiber becomes long and thin. Electron microscopy has resolved it into a bundle of dark-edged 150 to 180 A filaments which extends between "centriolar plaques" that are attached to the nuclear envelope.

The contents of bacterial cells can be reversibly pulled away. (plasmolyzed) from the wall by strong salt solutions. Bacterial "protoplasts"—living cells divested of their wall—shrink or swell with changing concentrations of solutes in their environment and burst when this concentration falls below a critical level. These observations have long suggested that the cytoplasm of bacteria is covered by a selectively permeable plasma membrane. In recent years electron microscopy has revealed the existence of a narrow profile at the surface of the cytoplasm of bacteria. In some bacteria, these profiles have the dimensions of sections of the "unit membrane" found in the interior and at the surface of plant and ainimal cells. In the place of a unit membrane, single tracks of densely stained granules are found in many bacteria. New electron micrographs confirm the observation of Young and Fitz-James (1959) that the first spore membrane is derived fromn an infolding of the plasma membrane of the mother bacterium. Illustrations are provided of the plasmna membrane of B. mycoides and of the yeast Lipomyces lipofer .

Sections of germinating spores of Bacillus polymyxa show that the primordial cell wall consists of a single layer. The intermediate layer and an outer rectangular array of macromolecules found on vegetative cells do not appear until the spore coats crack open about 60 min after initiation of germination. The initial areas of the new components appear in patches under the cracks in the coats. Within 10 min the wall is completed and takes on the profile seen in the vegetative cell. Negative staining and freeze-etching techniques show the regular structure to be identical with that previously shown for mature cells, although the subunits are more readily visible in negatively stained preparations.